This invention relates to an image processing apparatus and an image processing method as well as a computer program, and more particularly to an image processing apparatus and an image processing method as well as a computer program wherein image data which suffers from a false color caused, for example, by aberration of a lens are corrected to produce image data of a high quality.
When an image is picked up by a camera, various problems are caused by aberration of a lens. Particularly where a camera which includes a lens having a high magnification or a lens produced by a low production cost is used to pick up an image, the picked up image sometimes suffers from a false color caused by aberration of the lens. Among such false colors, a false color of purple having a very high color saturation sometimes appears in the proximity of an edge of a low luminance region when an image of such a scene that the luminance difference is very great (for example, a scene in which a beam of the sun leaks through leaves of trees) is picked up. This is called purple fringe (PF) and has a bad influence on the picture quality.
This is a phenomenon that a false color is generated at an edge portion in an image because the point image distribution differs depending upon the wavelength. Even if a false color is not very conspicuous with ordinary pixels, if the image includes an edge portion of a high contrast at which an overexposed highlight by a saturation condition in luminance level occurs, then a purple false color appears around the edge portion and this makes the image different from a natural image. Usually, since a purple-like color appears frequently as a false color in the proximity of an overexposed highlight, such a false color as just mentioned is called purple fringe. However, depending upon the lens, image pickup condition and so forth, a greenish color sometimes appears as a false color. In this manner, various false colors possibly appear. In the following description, the term “purple fringe” is used to signify a phenomenon of appearance of a false color at an edge portion of a high contrast at which an overexposed highlight appears irrespective of the color generated.
As a countermeasure for reducing the chromatic aberration, a configuration is available wherein a lens made of a special material such as fluorite is applied. However, in order to produce such a lens as just mentioned, a high cost is required, and consequently, the produced lens becomes expensive. Therefore, the lens of the type described is not popularly used although it is used in some high-grade cameras such as a camera of the lens exchange type.
Japanese Patent Laid-Open No. 2003-60983 discloses a method of reducing false colors, which are generated by chromatic aberration, through an image process. According to the method, a process for suppressing the color, that is, a process for decreasing the saturation, is performed for a region of an image in which a high frequency component of a green channel has a high level. Further, as a countermeasure where an overexposed highlight appears, a configuration that two images are picked up with different exposures and an original luminance of an overexposed highlight is estimated to lower the chromatic saturation of the overexposed highlight is disclosed in Japanese Patent Laid-Open No. 2003-60983.
However, according to the process disclosed in Japanese Patent Laid-Open No. 2003-60983, since it is decided from a result of estimation of the luminance of an overexposed highlight by what degree the chromatic saturation can be decreased, there is the possibility that, if the estimation is made in error, then the original color of the image pickup subject may drop excessively. As a result, an unnatural resultant image may be formed. Further, it is necessary to perform image pickup twice with different exposures in order to allow luminance estimation of an overexposed highlight, and there is a problem that, if a camera is shaken by a hand, blurring of an image pickup subject or the like occurs, and it is difficult to obtain a correct result. Further, since the exposure must be changed to perform image pickup twice, there is a problem that the method cannot be applied where image pickup of moving pictures is performed using a video camera or the like.